Our objective is to understand of the role of endogenous prostaglandins (PGs) in the effects of parathyroid hormone (PTH) on bone. Intermittently injected PTH is anabolic, while continuously infused PTH is catabolic. PTH is a potent inducer of cyclooxgenase-2 (COX-2), the enzyme primarily responsible for acute PG production. Similar to PTH, PGE2 acts via G-protein coupled receptors (GPCRs) to stimulate bone resorption and formation, and when injected in vivo, PGE2 can increase bone mass. The absence of COX-2 expression was expected to decrease PTH-stimulation of both osteoblast (OB) and osteoclast (OC) differentiation. However, we found that continuous treatment with PTH stimulates OB differentiation and mineralization in vitro in the absence of COX-2 expression or COX-2 produced PGs. In vivo, male COX-2 knockout (KO) mice have increased bone turnover and increased anabolic responses to intermittent PTH compared to wild type (WT) mice. Our general hypotheses are that PTH-induced PGs inhibit the stimulatory effects of PTH on mesenchymal progenitor differentiation into OBs and on mineralization by more mature OBs;that PG accumulation in the media accounts for the lack of osteogenic effects of continuous PTH in vitro;and that sustained elevation of PG production in vivo will both decrease the anabolic and increase the catabolic responses to PTH. In vitro, we will explore GPCR-signaling pathways and differential gene expression associated with the stimulatory effects of PTH and PGE2 alone on OB-differentiation and the inhibitory effects of endogenous or exogenous PGs on PTH-stimulated OB differentiation in marrow stromal cell (MSC) and primary osteoblast (POB) cultures. In vivo, we will use intermittent PTH and continuous PTH infusion to compare effects on anabolic and catabolic bone responses to PTH of global Cox-2 disruption and of Cox-2 deletion targeted to early and late OBs. Because continuous exposure to PTH becomes osteogenic in the absence of COX-2 expression in vitro, we propose that this may be true in vivo as well. Endpoints will be radiographic and histolomorphometric measures of skeletal phenotype, markers of bone turnover, and differential gene expression. This study is an opportunity to explore mechanisms by which two agents, both of which can be anabolic, produce a negative interaction, in an effort to understand the specific pathways involved. A better understanding of these mechanisms could have an important impact on our understanding of anabolic pathways and could have important clinical applications.